This study investigates the morphology and thermo-mechanical properties of cross-linked polyethylene (PEX) pipes for potential use in high-temperature borehole thermal energy storage systems. Particular attention is given to a novel type of PEX pipe produced through photoinitiated cross-linking (PEX-e). Two formulations, PEX-e1 and PEX-e2, were analyzed and compared to peroxide-cross-linked polyethylene (PEX-a) and non-cross-linked bimodal polyethylene (PE100) pipes. The degree of cross-linking was evaluated via gel content, while cross-link density and molecular weight between cross-links were determined using dynamic mechanical analysis (DMA). Phase composition and molecular mobility were explored through 1H static nuclear magnetic resonance (NMR), and the melting and crystallization behavior was assessed by differential scanning calorimetry (DSC). Oxidative stability and degradation were examined by using Fourier transform infrared (FTIR) spectroscopy, oxidation induction time (OIT) measurements, and thermogravimetric analysis (TGA). Both PEX-e formulations achieved satisfactory cross-linking degrees and exhibited remarkable OIT values. However, significant differences in cross-link distribution were noted, with PEX-e2 showing a less uniform dispersion of cross-links, which resulted in a lower storage modulus. FTIR analysis indicated that oxidation products were formed in PEX-e1 during cross-linking, highlighting the need for further optimization of the formulation and processing conditions.

This study presents a comprehensive evaluation of the thermo-oxidative stability of crosslinked polyethylene (PEX) pipes designed for borehole thermal energy storage (BTES) systems, with a particular focus on a novel PEX type produced via a photo-initiated crosslinking process (PEX-e). Two formulations, PEX-e1 and PEX-e2, were assessed and compared to commercial peroxide-crosslinked polyethylene (PEX-a) and bimodal polyethylene (PE100) pipes. The pipes were aged in distilled water at the intended BTES service temperature for 210 days, with periodic analyses conducted to monitor antioxidant (AO) depletion and the formation of degradation products. Advanced analytical techniques were employed, including Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy, gel content analysis, colorimetry, differential scanning calorimetry (DSC), oxidation induction time (OIT), and dynamic mechanical analysis (DMA). FTIR analysis revealed the presence of degradation-related peaks on the pipe surfaces, while the formation of oxidation products in the bulk material was limited. Phase composition analysis showed that physical ageing dominated during the first 30 days, leading to increased crystallinity and enhanced lamellar thickness. Over time, chain scission emerged as the primary degradation mechanism, resulting in molecular weight reductions, with PEX-a being the most severely affected. No abrupt changes in phase composition or mechanical properties were observed after 210 days of ageing, indicating that the pipes remained in the induction phase of degradation. Interestingly, despite their high OIT levels, both PEX-e formulations exhibited accelerated AO depletion during prolonged exposure, suggesting the need for further optimization of PEX-e formulations to ensure long-term stability under demanding BTES conditions. 

Borehole engineering encompasses the part of mining that involves the process of drilling boreholes and their utilization (e.g., for research, exploration, exploitation, and injection purposes). According to legal regulations, mining pits must be closed after their use, and this applies to pits in the form of boreholes as well. The Laboratory of Geoenergetics at AGH University of Krakow is involved in adapting old, exploited and already closed boreholes for energetic purposes. This includes geothermal applications, as well as energy storage in rock formations and boreholes. Geoenergetics is a relatively new concept that combines geothermal energy with energy storage in rock formations (including boreholes). One type of analysed borehole is a freezing borehole. They are used, for example, in drilling mining shafts that are in the vicinity of aquifers and are drilled using the rotary drilling method with a reverse circulation of drilling mud, or in peat bogs. For borehole heat exchangers based on freezing boreholes for long-term mathematical modelling, several heating scenarios were considered with several thermal loads. The maximum average power obtained after one year of usage of four boreholes with variable temperatures was 11 kW. With the usage of 10 boreholes the power reached over 27 kW. The heat-carrying temperature was assumed to be 22 °C during early summer (June and July) and 2 °C during the rest of the year. When considering stable exploitation during a 10-year period with four boreholes with the same temperatures, a heating power of over 12 kW was obtained, as well as a power of over 28 kW when considering using 10 boreholes. The maximum amount of heat obtained during the 10-year period using 10 boreholes was over 8.8 thousand GJ. Once they have fulfilled their function, these boreholes lose their technological significance. In the paper, the concept is outlined, and the results of the analysis are described using the numerical program BoHEx.

Polyethylene (PE) is a low-cost and versatile material for pipe applications. However, when it comes to at elevated temperatures, the use of PE is restricted due to its propensity to crack under stress and to undergo thermo-oxidative degradation reactions (ageing)1. Cross-linked polyethylene (PEX) has been appointed as a promising alternative to solve these limitations1. There are several different cross-linking processing methodologies for polyethylene pipes. The most usual methods are based on peroxide (PEX-a), silane (PEX-b) and irradiation (PEX-c). Additionally, photo-induced cross-linking (PEX-e), has recently garnered attracted significant commercial interest, but there is still very little information in the literature regarding this novel material. Recent findings appoint to hot water as being the key component element driving the ageing mechanism in PEX-a pipes used for hot water transportation. However, further investigation of the effects of ageing on PEX properties over time is of high importance. Accordingly, the present work aims to evaluate the properties of different types of PEX pipes as a function of ageing. PEX-a, two formulations of PEX-e and PE pipe samples were submersed in distilled water at 95°C for 150 days. Samples were removed periodically and the effect of degradation on the properties was measured by differential scanning calorimetry (DSC), oxidation induction time (OIT), dynamical mechanical thermal analysis (DMA) and tensile tests. Initial results showed that the OIT values for PEX-a before ageing were considerably lower than for the other materials but remained overall the same after 60 days of ageing. For the two formulations of PEX-e, a considerable decrease in OIT was observed, indicating that stabilizing additive hydrolysis was high for this PEX type under these conditions.

In this study, soapstone waste originated from craftsmanship activities was used as an alternative filler (0–30 wt%) for a high-density polyethylene (PE) matrix. The aim of this paper is to understand the effect of the filler particles on crystallinity, thermal stability and thermo-mechanical properties of this newly developed composite material. Physico-chemical characterization was performed by x-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy. Thermogravimetric analysis (TGA), oxidation induction time (OIT) and dynamic mechanical thermal analysis (DMA) were performed to assess the effect of the filler on the themo-mechanical properties of PE. Thermal stability, measured by TGA, was enhanced, while OIT values reduced with filler content. A significant increase on the storage modulus of the composites (up to 148% in comparison with unfilled PE) was observed and this reinforcing effect was even more prominent at higher temperatures. XRD analysis revealed that the degree of crystallinity improved significantly with soapstone loading, which explains the substantial increase in stiffness observed. Increased crystallinity is also associated with higher strength, reduced residual stress, and better dimensional stability of end products, which can be particularly attractive for pressure pipe applications. 

An experimental study of high-density polyethylene (HDPE) composites filled with talc (0–15 wt.%) was carried out to investigate the rheological properties. The apparent melt viscosity, melt density, and die-swell ratio (B) of the composites were measured at constant shear stress and constant shear rate by using a melt flow indexer and capillary rheometer. The experimental conditions were set to a temperature range from 190 to 220 C for both apparatuses whereas a load range from 5 to 12.16 kg was selected for melt flow indexer and shear rate range from 1 to 10000 s1 for capillary rheometer. The initial study showed that the talc particulates did not influence the melt viscosity compared with the neat HDPE but decreased the elasticity of the polymer system. The HDPE/talc systems obeyed power-law model in shear stress–shear rate variations and were shear thinning, meanwhile, the die-swell increased with an increased wall shear rate and shear stress. The melt density of the composites increased linearly with an increase of the filler weight fraction and decreased with the increase of the testing temperature. The talc-HDPE composites showed compressible in the molten state.

When considering implementation of shallow geothermal energy as a renewable source for heating and cooling of buildings, special care should be taken in the hydraulic design of the borehole heat exchanger system. Laminar flow can occur in pipes due to the usage of glycol mixtures at low temperature or inadequate flow rates. This can lead to lower heat extraction and rejection rates of the exchanger because of higher thermal resistance. Furthermore, by increasing the flow rate to achieve turbulent flow and satisfactory heat transfer rate can lead to an increase in the pressure drop of the system and oversizing of the circulation pump which leads to impairment of the seasonal coefficient of performance at the heat pump. The most frequently used borehole heat exchanger system in Europe is a double-loop pipe system with a smooth inner wall. Lately, development is focused on the implementation of a different configuration as well as with ribbed inner walls which ensures turbulent flow in the system, even at lower flow rates. At a location in Zagreb, standard and extended thermal response tests were conducted on three different heat exchanger configurations in the same geological environment. With a standard TRT test, thermogeological properties of the ground and thermal resistance of the borehole were determined for each smooth or turbulator pipe configuration. On the other hand, extended Steady-State Thermal Response Step Test (TRST) incorporates a series of power steps to determine borehole extraction rates at the defined steady-state heat transfer conditions of 0/−3 °C. When comparing most common exchanger, 2U-loop D32 smooth pipe, with novel 1U-loop D45 ribbed pipe, an increase in heat extraction of 6.5% can be observed. Also, when the same comparison is made with novel 2U-loop D32 ribbed pipe, an increase of 18.7% is achieved. Overall results show that heat exchangers with ribbed inner pipe wall have advantages over classic double-loop smooth pipe designs, in terms of greater steady-state heat extraction rate and more favorable hydraulic conditions.

A ground source heat pump (GSHP) system can be used for both cooling and heating modes simultaneously for commercial, industrial and residential buildings virtually at any location with great flexibility to cover a wide range of demands all around the world. Polyethylene (PE) has been used as the main raw material in production of the Ground Heat Exchangers (GHE). This paper briefly reviews the history of polyethylene and development in polymerization process with emphasis on the third-generation bimodal structure. The characteristics of PE pipes used in GSHP systems are discussed. This paper is devoted to a critical review on the attempts in post-polymerization treatments of the PE, and GHEs to improve the performance of the systems. The experimental and simulated comparisons show that the enhancement of the thermal conductivity of the material can reduce significantly the overall borehole thermal resistance.